This invention relates to a composition comprising a photo-catalytic metal oxide.
The photo-catalytic metal oxide-containing composition of the invention is characterized in that the photo-catalytic activity is maintained at a sufficiently high level, the viscosity increase with time is extremely reduced and the workability is excellent, and thus, the photo-catalytic composition is suitably coated as a film-forming material on a base material.
In recent years, extensive researches have been made for utilizing the photo-catalytic action of metal oxides such as titanium oxide. The photo-catalytic metal oxide has excellent functions, for example, of preventing pollution by removing harmful substances, of decomposing ammonia and sulfur compounds in air for purification, and of sterilization. Thus, the use of photo-catalytic metal oxides is being expected in various fields. Particles of the photo-catalytic metal oxide are utilized in various forms depending upon the particular intended use thereof, such as bulk particles, a sol and a film. In view of the practical use, the photo-catalytic metal oxide particles are made into a film in many cases because of ease in handling.
As the method of forming photo-catalytic metal oxide particles into a film, a sol-gel method is known wherein a photo-catalytic metal oxide film is formed on a surface of glass sheet as a base material, from a metal alkoxide raw material. The sol-gel method is, however, disadvantageous in that the starting material is expensive and the working environment is polluted and a problem arises in safety because a large quantity of an organic solvent is used. Further, the process of firmly bonding a film of the photo-catalytic metal oxide to the base material must be performed at a high temperature. Thus, a base material used for a sol-gel method must be heat-resistant and this is an obstacle to enlargement of the use.
Attempts have been proposed for bonding photo-catalytic metal oxide particles onto a base material through a binder such as a film-forming composition comprising a hydrolysate of a hydrolyzable silicon compound as a binder in Japanese Unexamined Patent Publication No. H8-164334 (Japanese unexamined patent publication is hereinafter abbreviated to xe2x80x9cJP-Axe2x80x9d), and a coating composition comprising a film-forming material comprising a siloxane polymer or silicate, and a hydrolyzable silane derivative monomer as a binder in JP-A H9-188850. These proposed compositions contain a hydrolyzable silicon compound, which is gradually hydrolyzed to allow a polycondensation reaction to proceed. As a result, the composition undergo decrease in adhesion strength or increase of viscosity during storage, and thus, the pot life is undesirably shortened.
A coating composition comprising titanium oxide having photo-catalytic activity, a hydrolyzable organic metal compound, and an aqueous solvent has been proposed in JP-A H9-40872. The organic metal compound is subject to hydrolysis, and thus, the increase in viscosity of the composition and the reduction of adhesion strength thereof occur during storage of the composition. Therefore, the proposed composition must be used as a two-pack type composition, and the addition of water and a catalyst must be carried out immediately before the coating, and thus, the handling property is poor.
Various attempts have heretofore been made to use a film of a photo-catalytic metal oxide by fixing the film to be supported on a base material such as glass or plastic material. However, the level of properties required for the fixed photo-catalytic metal oxide film is high, and photo-catalytic metal oxide films capable of satisfying the requirements have not yet been found.
An object of the present invention is to provide a composition comprising a photo-catalytic metal oxide capable of maintaining the photo-catalytic activity at a sufficiently high level, exhibiting an increase of viscosity with time only to a negligible extent, having good workability, and exhibiting good adhesion when the composition is coated as a film-forming material on a base material.
Another object of the invention is to provide a film made from the above-mentioned photo-catalytic metal oxide-containing composition.
Still another object of the invention is to provide a composite of a photo-catalytic metal oxide film with a base material, which is formed by coating a surface of the base material with the photo-catalytic metal oxide-containing composition, and is characterized as being transparent and possessing high adhesion between the film and the base material whereby the film is not easily separated from the base material.
As a result of extensive investigations for photo-catalytic metal oxide-containing compositions using various film-forming materials, the present inventors have found that a photocatalytic metal oxide-containing composition having incorporated therein a specific zirconium compound is suitable as a film-forming material capable of maintaining the photocatalytic activity at a sufficiently high level, exhibiting an increase of viscosity with time only to a negligible extent, having good workability, and exhibiting good adhesion when the composition is coated as a film-forming material on a base material.
Thus, in one aspect of the present invention, there is provided a composition comprising metal oxide particles having photo-catalytic activity, a solvent-soluble and non-hydrolyxable zirconium compound, and a solvent.
In another aspect of the present invention, there is provided a film made from the above-mentioned composition comprising the photo-catalytic metal oxide particles.
In a further aspect of the present invention, there is provided a composite of a film with a base material, said film being formed from the above-mentioned composition comprising the photo-catalytic metal oxide particles.
The photo-catalytic metal oxide particles for use in the present invention comprise particles of a metal oxide exhibiting a catalytic activity for oxidation-reduction reaction of an organic compound when the metal oxide particles are irradiated with light having an energy larger than the band gap between the conduction band and the valence band of the crystal.
As specific examples of the photo-catalytic metal oxide particles, there can be mentioned particles of rutile titanium oxide, anatase titanium oxide, brookite titanium oxide, strontium titanate, tin oxide, zinc oxide and iron oxide.
The photo-catalytic reaction occurs in the vicinity of the surface of the metal oxide particle, and accordingly, the metal oxide particle preferably has a small particle size. However, those having an average particle diameter of smaller than about 0.005 xcexcm are difficult to produce, therefore, the average particle diameter is usually in the range of about 0.005 to about 0.3 xcexcm, preferably from about 0.01 to about 0.1 xcexcm. The specific surface area is preferably in the range of about 50 to about 300 m2/g.
Of the photo-catalytic metal oxides, titanium oxide is most preferably because it is commercially available as finely divided particles, and harmless and chemically stable. Any conventional titanium oxide prepared by known methods can be used in the invention. In the case where titanium oxide is prepared by hydrolysis, the titanium oxide-containing sol prepared by hydrolysis can be used. Alternatively, the titanium oxide-containing sol is formed into a finely divided titanium oxide powder, which is then dispersed in a liquid medium to prepare a photo-catalytic oxide composition. Finely divided titanium oxide particles may also be prepared by a vapor phase method. Especially preferably finely divided titanium oxide particles are prepared by a method of hydrolyzing titanium chloride or titanium sulfate because the operation is easy and the cost is low.
The hydrolysis method is not particularly limited, but it is preferable that hydrolysis is performed in a hydrolysis vessel equipped with a reflux condenser while the gas evaporated is condensed to flow back into the vessel. The finely divided titanium oxide particles have good dispersibility and a film made therefrom exhibits good transparency.
The amount of the photo-catalytic metal oxide in the composition is preferably in the range of about 1 to about 25% by weight based on the total weight of the composition. If the amount is smaller than about 1% by weight, the photo-catalytic activity is poor. In contrast, if it exceeds about 25% by weight, the dispersibility of metal oxide particles is reduced to cause undesirable increase in viscosity of a coating solution, and the composition becomes unstable.
The zirconium compound for use in the present invention exhibits a function of binder when the photo-catalytic metal oxide-containing composition is coated on a base material. The zirconium compound is not particularly limited provided that it is soluble in water or an organic solvent, and non-hydrolyzable. By the term xe2x80x9cnon-hydrolyzablexe2x80x9d used herein, we mean that the zirconium compound is not hydrolyzed during the preparation and storage of the composition of the present invention.
As specific examples of the zirconium compound, there can be mentioned acidic aqueous solution-forming zirconium compounds such as zirconium oxychloride, zirconium hydroxychloride, zirconium nitrate, zirconium sulfate and zirconium acetate; alkaline aqueous solution-forming zirconium compounds such as ammonium zirconium carbonate, potassium zirconium carbonate and sodium zirconium phosphate; and organic solvent-soluble zirconium compounds such as zirconium propionate. Especially water-soluble zirconium compounds such as zirconium oxychloride, zirconium hydroxychloride, zirconium nitrate and ammonium zirconium carbonate are preferable because an organic solvent is not used for the preparation of a solution, and the working environment is not contaminated. A complex or complex salt of a zirconium compound having at least one group selected from hydroxyl group and carbonate group, and a high-molecular-weight zirconium compound thereof may also be used. The high-molecular-weight zirconium compound is commercially available. For example, it is supplied by Magnesium Elektron Ltd. (see xe2x80x9cData Sheet 150xe2x80x9d, published Sept. 1985 by the same company).
The solvent-soluble and non-hydrolyzable zirconium compounds may be used either alone or as combination of at least two thereof.
When an aqueous dispersion of the photo-catalytic metal oxide composition in the form of a sol if used, if the sol of composition is acidic, it is preferable to use a zirconium compound having acidity, or if the sol of composition is alkaline, a zirconium compound having alkalinity is preferably used, so that pH shock can be avoided.
The amount of the zirconium compound to be incorporated in the composition is preferably in the range of about 3 to about 200 parts by weight, calculated as the weight of ZrO2, per 100 parts by weight of the photo-catalytic metal oxide particles. If the amount of the zirconium compound as ZrO2 is smaller than about 3 parts by weight, the film formed on a base material is fixed to the base material with a weak bond a strength. In contrast, if the amount of the zirconium compound as ZrO2 is larger than about 200 parts by weight, the photo-catalytic activity is reduced to a great extent. When titanium oxide particles are used as the photo-catalytic metal oxide particles, the degree of variation of the contact angle of the film occurring upon irradiation of light can be controlled by appropriately choosing the proportion of the zirconium compound to the titanium oxide particles. The amount of the zirconium compound as ZrO2 is more preferably in the range of about 5 to about 60 parts by weight.
The solvent for use in the present invention is at least one selected from water and organic solvents. Among organic solvents, those which are hydrophilic are preferable. As specific examples of the hydrophilic organic solvents, there can be mentioned monohydric alcohols such as methanol, ethanol, propanol and butanol, polyhydric alcohols such as ethylene glycol, diethylene glycol and glycerin, ketones such as acetone and methyl ethyl ketone; esters such as ethyl acetate and butyl acetate; and cellosolves such as ethyl cellosolve. These solvents may be used either alone or as a combination of two or more thereof. A suitable solvent is determined preferably by taking account of the stability of the composition, the kind of a base material, the drying conditions employed at the formation of a film, and the production cost.
The composition of the present invention comprises metal oxide particles having photo-catalytic activity, a solvent-soluble zirconium compound and a solvent. Various additives may be incorporated in the composition. For example, to enhance the film-forming property of the composition, a thickening agent such as glyoerin, polyvinyl alcohol, methyl cellulose and polyethylene glycol, and a dispersant such as polyacrylic acid and polymethacrylic acid may be incorporated. Metal particles such as silver or copper may be added to impart to the composition a sterilizing activity exhibited even in a dark place. A platinum group metal such as palladium or platinum may be added to enhance the photo-catalytic activity. These additives are incorporated each in an amount of approximately in the range of 10 to 10,000 ppm.
The composition of the present invention can be prepared by mixing together the photo-catalytic metal oxide particles, the solvent-soluble zirconium compound, the solvent, and optional additives described above, with stirring. The as-prepared composition can be stored as it is.
The composition of the present invention is coated on a base material made of various materials or molded articles to form a photo-catalytic metal oxide film on the surface of the base material. As specific examples of the base material, there can be mentioned glass, plastics, ceramics, metals, wood, paper and concrete. Especially glass is preferable. An electric lamp and lighting fixtures can also be a base material for coating. In the case where these molded articles are used, the solvent contained in the composition is preferably a mixed solvent composed of water and an organic solvent.
The thickness of the film formed on the base material is preferably in the range of about 0.05 to about 2 xcexcm. If the film thickness is smaller than about 0.05 xcexcm, the photo-catalytic activity is poor. In contrast, if the film thickness exceeds about 2 xcexcm, the film is readily separated from the base material, and such a large thickness is not preferred from a cost consideration.
The composition of the present invention can be coated on a base material by any known methods. For example, spin coating, flow coating, dip coating, spray coating, bar coating and brush coating can be employed. To prevent deterioration of the base material by the photo-catalytic metal oxide particles, an undercoat composed of silica or a fluororesin can be formed as an intermediate layer between the film and the base material, prior to the coating of the film.
After coating, the film is cured, for example, by allowing the film to stand at room temperature or subjecting it to heat treatment. The curing temperature is in the range of about 20 to about 200xc2x0 C., preferably about 20 to about 150xc2x0 C. The curing time varied depending upon the particular solvent used, but is usually about 10 minutes when the film is cured, for example, at about 100xc2x0 C. When a heat-resistant base material is used, the curing can be performed at a high temperature of about 200 to about 400xc2x0 C. to sinter the photo-catalytic metal oxide particles.
The invention will now be specifically described by the following examples that by no means limit the scope of the invention.
Water was added to titanium tetrachloride (purity: 99.9%) to prepare an aqueous titanium tetrachloride solution having a concentration of 0.25 mol/l (2% by weight as expressed in terms of titanium dioxide). During the preparation of the solution, the aqueous solution was ice-cooled by a cooling device so that the temperature of the aqueous solution did not exceed 50xc2x0 C. Then 1 liter of the aqueous solution was charged in a reaction vessel with a reflux condenser, heated to a temperature (104xc2x0 C.) in the vicinity of the boiling point, and maintained at this temperature for 60 minutes to allow the hydrolysis to proceed, thereby preparing an aqueous dispersion of titanium oxide sol. The sol was concentrated by decantation and then an aqueous dispersion of titanium oxide sol having a titanium oxide concentration of 25% by weight and a pH value of 5.5 was obtained by using an electrodialyser G3 may by Asahi Chemical Industry Co., Ltd. A part of the acidic titanium oxide sol was sampled and its particle diameter was measured by an electrophoretic light scattering photometer. The acidic titanium oxide sol was proved to have an average particle diameter of 0.05 xcexcm and a specific surface area of 122 m2/g.
Using this acidic titanium oxide sol, coating solutions (coating composition) were prepared as follows.